Strand casting, also known as continuous casting, involves a process in which molten metal is introduced into a mold. During its residence time in the mold, the metal solidifies in contact with the wall of the mold and can be drawn downward via a movable bottom portion of the mold. A reserve of molten metal is positioned above the mold in what is referred to as a feeder head or a “hot-top”. A parting agent, or release agent, is applied to the surface of the continuous casting being formed, such that direct contact with the surface of the mold is avoided, thereby facilitating easy removal of the casting. Parting agents may include mixtures of oils and gases. It is particularly desirable if the oil-gas mixture is first formed close to the mold.
Hot-top molds of the kind described above are well known in the art. U.S. Pat. No. 5,320,159 to Schneider et al. discloses a hot-top mold where a parting agent, by way of a parting agent distributor, reaches the surface of the cast strand. Two different parting agents such as oil and gas, may be fed separately or as a mixture.
Certain hard-to-cast alloys, such as aluminum alloys containing lead, zinc, tin and copper, pose casting problems that can result in poor surface quality. Recently, such alloys have been gaining importance in the production of special alloys and machining alloy stock, which are to be used at a high cutting speed. Additionally, when the parting agent supply is insufficient or not uniformly regulated, surface and edge structural defects can develop in the casting. These include, in particular, surface irregularities or in-homogeneities in the structure near the surface.
One problem which contributes to the surface irregularities spoken about above and determines whether the parting agent reaches the entire surface of the metal strand is the precise control of the gas pressure. Pressure fluctuations can result in surface flaws, and pressure that is too high may pose a risk that gas might escape through the molten metal.
U.S. Pat. No. 4,732,209 to Pechiney attempts to solve this problem by using a graphite ring on the inside of the mold and a gas under pressure that is forced from the outside through the open pores of a feeder head to the inside of the mold and thereby acts as a parting agent between the surface of the forming metal strand and the mold surface. However, while this solution addresses the problem of controlling gas pressure, this solution cannot be applied to hard-to-cast alloys.